The present invention relates to manholes and more specifically to new manhole designs that give rise to stronger and less costly manholes compared to conventional manholes.
The art of manhole design is about one century old. Today, manholes come in a vast variety of shapes, sizes, and materials and serve a great many purposes. A manhole comprises three basic parts, namely, a closed skirt that at least partially embeds in the ground or concrete surface layer, a closed ring connected to the top of the skirt and a lid or cover that removably cooperates and permits access to the interior of the chamber defined by the skirt.
With reference to FIGS. 3 and 4, a typical conventional, circular manhole 10 is shown that comprises a cylindrical skirt 12, a closed cast metal or rolled steel top ring 14, and a removable circular metal lid 16. Skirt 12 is commonly formed from flexible sheet steel, cut and welded at a seam to form the cylinder. Flexible skirted manholes of this type were first introduced in about 1975. Top ring 14 is typically made in two circular parts, closed L-shaped lower ring 17 and cylindrical top ring 18. Ring 17 is welded to the top of skirt 12 and ring 18 is welded to the top of ring 17, generally as shown. Cover 16 rests on ring 17 when in the closed position.
In practice, manholes are fabricated at a works location and delivered with the skirt 12 and ring 14 welded in place. Installation includes usually preparing a bed of small stones 20 below ground surface, placing the manhole with the bottom rim of skirt 12 resting on stones 20 so that the top of ring 18 defines a predetermined ground plane or surface above stones 20. Concrete 22 is then poured on to stones 20 to a predetermined depth, such as a standard 8 or 16 inches.
Conventional manholes create a number of problems in the field. For example, the skirts of manholes greater than 24 inches in diameter provide insufficient lateral strength giving rise to bending or other deformation of the skirt during handling. For example, manholes are often placed or impacted on their sides during delivery from a truck or flatbed trailer, thereby resting on or impacting one point of the bottom of skirt 12. The weight of the parts or impacting lateral force thus deforms skirt 12 causing deformation of the top ring, problems with installation of the manhole itself, or the placement of equipment to be housed in the chamber.
In addition, the bottom rim of skirt 12 tends to penetrate the bed of small stones 20, thus reducing the vertical distance available for the thickness of concrete 22. This sometimes leads to the concrete depth being below required specifications which is very costly to correct.
Also, the conventional fabrication of the rings 17 and 18 on the top of skirt 12 requires many process steps and welds or connections, thus increasing the manufacturing costs of the assembly.
Another problem results during installation after the manhole is in place. The flow of concrete laterally against the skirt, if not carefully controlled, exerts lateral forces that bend the bottom of the skirt inward resulting in a distortion of the circular top ring. In this case, if the manhole cover is on during concrete pour, the ring distortion locks it on and the cover cannot be removed. If it is forced off, it will not re-seat due to the distorted ring.
Thus, there is a longfelt need in the art of making manholes with rolled steel or flexible skirts to solve the foregoing problems without increasing the cost or weight characteristics of the manhole. In addition, weight and cost benefits would result from the redesign of the double top ring manhole design to a single top ring.